1. Field of the Invention
The present invention relates to a speaker having a function for delivering a signal that is proportionate to vibration of the diaphragm.
2. Description of the Prior Art
In order to see the state of a vibrating speaker diaphragm, it is sometimes necessary to pick up a signal that is proportionate to vibration of the diaphragm. A conventional speaker provided with a detection coil wound around the voice coil bobbin for obtaining such proportionate signal is illustrated in FIG. 8.
The magnetic circuit 5 of the speaker is formed of a ring-shaped magnet 3 disposed on a plate 2 having a center pole 1, and an upper plate 4. A frame 11 is connected on the magnetic circuit 5, and a cone-shape diaphragm 13 is adhered to the peripheral part of the frame 11 via an edge 12. A voice coil bobbin 9 wound around with a voice coil 7 and a detection coil 8 is connected to the diaphragm 13 at the center the bobbin, at its middle part, is also connected to the frame 11 via a damper 10. The diaphragm 13 is attached in the center with a dust cap 14. Terminals 16, 18 provided on the frame 11 are attached respectively with flexible wires 15, 17; the respective other ends of the flexible wires 15, 17 are connected to the voice coil 7 and the detection coil 8 at a place in the middle of the voice coil bobbin 9.
When an electric signal is applied to the terminal 16, the voice coil 7 disposed in a gap 6 of the magnetic circuit 5 moves in accordance with Fleming""s left-hand rule to vibrate the diaphragm 13, which is connected with the voice coil bobbin 9. As a result, the diaphragm 13 generates a sound. As a result, in accordance with Fleming""s right-hand rule, an electric signal is induced in the detection coil 8 in proportion to the motion of voice coil 7. The electric signal is delivered outside through the terminal 18.
In the conventional speaker of the above configuration, which has a detection coil 8 wound around voice coil bobbin 9 for delivering the proportionate signals out, both the detection coil 8 and the voice coil 7 are formed with a same diameter wire having round cross section in two winding layers respectively, as illustrated in FIG. 9. This structure makes the outer diameter of the whole coil structure larger which eventually requires making the gap 6 proportionately wider. This causes problems; namely, the magnetic flux density in the magnetic gap 6 decreases, efficiency of the speaker deteriorates, furthermore, the Q0 (sharpness of resonance) increases.
The present invention addresses the above problems. A speaker of the present invention forms a detection coil with a wire thinner than one fourth the diameter of that of the voice coil wound around the bobbin by making use of a space existing among the coiled wire of the voice coil. Another speaker of the present invention forms both the voice coil and the detection coil with a flat type wire, for obtaining the same sound output and the same detection capability as the conventional speaker without inviting increased overall dimensions of the whole coil structure.
In accordance with the structure of the present invention, the outer diameter of a coil containing a voice coil and a detection coil does not increase, the density of magnetic flux in the magnetic gap does not decrease. Thus a signal that proportionately represents the vibration of diaphragm is made available without inviting deteriorated speaker efficiency and an increased Q0.